The performance of a transmitter depends upon the transmitted power level. As the transmitted power increases beyond a certain level, distortion degrades the transmitted signal. This distortion results in data loss, high bit error rates, and the like. To prevent these effects, power control schemes have been developed.
Two common power control schemes are open loop power control and closed loop power control. In both schemes the goal is to maintain a target output power level at the transmitter. However, both schemes have drawbacks. In either scheme, any known power level error must be subtracted from the target power level to avoid exceeding specified maximum power levels. This can result in unnecessarily low transmitted power levels. Furthermore, other factors such as temperature, voltage standing wave ratio (VSWR), and the like can cause large errors, especially for open loop power control schemes.
To directly address the distortion problem, pre-distortion schemes have been developed. According to these schemes, the distortion produced by the transmitter is measured, and then applied inversely to the source signal, before feeding the source signal to the transmitter. Both analog and digital pre-distortion schemes have been developed. FIG. 1 shows a conventional digital pre-distortion scheme.
Referring to FIG. 1, a digital signal source 102 produces a digital signal 104. A digital pre-distortion (DPD) module 106 pre-distorts digital signal 104 based on DPD information 108 provided by DPD training module 110. A digital-to-analog converter (DAC) 112 converts pre-distorted digital signal 114 to an analog signal 116. Transmitter 118 transmits a signal 120 that represents analog signal 116. Transmitter 118 transmits signal 120 at a power level specified by a gain setting 122 provided by a gain setting module 124. A receiver 126 receives signal 120 and produces a second analog signal 128 based on signal 120. An analog-to-digital converter (ADC) 130 converts second analog signal 128 to a second digital signal 132. DPD training module 110 produces DPD information 108 based on a comparison of digital signals 104 and 132.